RFC 3520

Network Working Group L-N. Hamer
Request for Comments: 3520 B. Gage
Category: Standards Track Nortel Networks
B. Kosinski
Invidi Technologies
H. Shieh
AT&T Wireless
April 2003
Session Authorization Policy Element
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
This document describes the representation of a session authorization
policy element for supporting policy-based per-session authorization
and admission control. The goal of session authorization is to allow
the exchange of information between network elements in order to
authorize the use of resources for a service and to co-ordinate
actions between the signaling and transport planes. This document
describes how a process on a system authorizes the reservation of
resources by a host and then provides that host with a session
authorization policy element which can be inserted into a resource
reservation protocol (e.g., the Resource ReSerVation Protocol (RSVP)
PATH message) to facilitate proper and secure reservation of those
resources within the network. We describe the encoding of session
authorization information as a policy element conforming to the
format of a Policy Data object (RFC 2750) and provide details
relating to operations, processing rules and error scenarios.
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RFC 3520 Session Authorization Policy Element April 2003
1. Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in BCP 14, RFC 2119
[RFC-2119].
2. Introduction
RSVP [RFC-2205] is one example of a resource reservation protocol
that is used by a host to request specific services from the network
for particular application data streams or flows. RSVP requests will
generally result in resources being reserved in each router along the
data path. RSVP allows users to obtain preferential access to
network resources, under the control of an admission control
mechanism. Such admission control is often based on user or
application identity [RFC-3182], however, it is also valuable to
provide the ability for per-session admission control.
In order to allow for per-session admission control, it is necessary
to provide a mechanism for ensuring use of resources by a host has
been properly authorized before allowing the reservation of those
resources. In order to meet this requirement, there must be
information in the resource reservation message which may be used to
verify the validity of the reservation request. This can be done by
providing the host with a session authorization policy element which
is inserted into the resource reservation message and verified by the
network.
This document describes the session authorization policy element
(AUTH_SESSION) used to convey information about the resources
authorized for use by a session. The host must obtain an
AUTH_SESSION element from an authorizing entity via a session
signaling protocol such as SIP [RFC-3261]. The host then inserts the
AUTH_SESSION element into the resource reservation message to allow
verification of the network resource request; in the case of RSVP,
this element MUST be encapsulated in the Policy Data object [RFC-
2750] of an RSVP PATH message. Network elements verify the request
and then process the resource reservation message based on admission
policy.
[RFC-3521] describes a framework in which a session authorization
policy element may be utilized to contain information relevant to the
network's decision to grant a reservation request.
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RFC 3520 Session Authorization Policy Element April 2003
3. Policy Element for Session Authorization
3.1 Policy Data Object Format
The Session Authorization policy element conforms to the format of a
POLICY_DATA object which contains policy information and is carried
by policy based admission protocols such as RSVP. A detailed
description of the POLICY_DATA object can be found in "RSVP
Extensions for Policy Control" [RFC-2750].
3.2 Session Authorization Policy Element
In this section we describe a policy element (PE) called session
authorization (AUTH_SESSION). The AUTH_SESSION policy element
contains a list of fields which describe the session, along with
other attributes.
+-------------+-------------+-------------+-------------+
| Length | P-Type = AUTH_SESSION |
+-------------+-------------+-------------+-------------+
// Session Authorization Attribute List //
+-------------------------------------------------------+
Length: 16 bits
The length of the policy element (including the Length and P-Type)
is in number of octets (MUST be in multiples of 4) and indicates
the end of the session authorization information block.
P-Type: 16 bits (Session Authorization Type)
AUTH_SESSION = 0x04
The Policy element type (P-type) of this element. The Internet
Assigned Numbers Authority (IANA) acts as a registry for policy
element types as described in [RFC-2750].
Session Authorization Attribute List: variable length
The session authorization attribute list is a collection of
objects which describes the session and provides other information
necessary to verify the resource reservation request. An initial
set of valid objects is described in Section 3.3.
3.3 Session Authorization Attributes
A session authorization attribute may contain a variety of
information and has both an attribute type and subtype. The
attribute itself MUST be a multiple of 4 octets in length, and any
attributes that are not a multiple of 4 octets long MUST be padded to
a 4-octet boundary. All padding bytes MUST have a value of zero.
Hamer, et al. Standards Track [Page 4]

RFC 3520 Session Authorization Policy Element April 2003
+--------+--------+--------+--------+
| Length | X-Type |SubType |
+--------+--------+--------+--------+
| Value ...
+--------+--------+--------+--------+
Length: 16 bits
The length field is two octets and indicates the actual length of
the attribute (including Length, X-Type and SubType fields) in
number of octets. The length does NOT include any bytes padding
to the value field to make the attribute a multiple of 4 octets
long.
X-Type: 8 bits
Session authorization attribute type (X-Type) field is one octet.
IANA acts as a registry for X-Types as described in section 7,
IANA Considerations. Initially, the registry contains the
following X-Types:
1 AUTH_ENT_ID The unique identifier of the entity which
authorized the session.
2 SESSION_ID Unique identifier for this session.
3 SOURCE_ADDR Address specification for the session
originator.
4 DEST_ADDR Address specification for the session
end-point.
5 START_TIME The starting time for the session.
6 END_TIME The end time for the session.
7 RESOURCES The resources which the user is authorized
to request.
8 AUTHENTICATION_DATA Authentication data of the session
authorization policy element.
SubType: 8 bits
Session authorization attribute sub-type is one octet in length.
The value of the SubType depends on the X-Type.
Value: variable length
The attribute specific information.
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RFC 3520 Session Authorization Policy Element April 2003
3.3.1 Authorizing Entity Identifier
AUTH_ENT_ID is used to identify the entity which authorized the
initial service request and generated the session authorization
policy element. The AUTH_ENT_ID may be represented in various
formats, and the SubType is used to define the format for the ID. The
format for AUTH_ENT_ID is as follows:
+-------+-------+-------+-------+
| Length |X-Type |SubType|
+-------+-------+-------+-------+
| OctetString ...
+-------+-------+-------+-------+
Length
Length of the attribute, which MUST be > 4.
X-Type
AUTH_ENT_ID
SubType
The following sub-types for AUTH_ENT_ID are defined. IANA acts as
a registry for AUTH_ENT_ID sub-types as described in section 7,
IANA Considerations. Initially, the registry contains the
following sub-types of AUTH_ENT_ID:
1 IPV4_ADDRESS IPv4 address represented in 32 bits
2 IPV6_ADDRESS IPv6 address represented in 128 bits
3 FQDN Fully Qualified Domain Name as defined in
RFC 1034 as an ASCII string.
4 ASCII_DN X.500 Distinguished name as defined in RFC
2253 as an ASCII string.
5 UNICODE_DN X.500 Distinguished name as defined in RFC
2253 as a UTF-8 string.
6 URI Universal Resource Identifier, as defined
in RFC 2396.
7 KRB_PRINCIPAL Fully Qualified Kerberos Principal name
represented by the ASCII string of a
principal followed by the @ realm name as
defined in RFC 1510 (e.g.,
principalX@realmY).
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RFC 3520 Session Authorization Policy Element April 2003
8 X509_V3_CERT The Distinguished Name of the subject of
the certificate as defined in RFC 2253 as a
UTF-8 string.
9 PGP_CERT The PGP digital certificate of the
authorizing entity as defined in RFC 2440.
OctetString
Contains the authorizing entity identifier.
3.3.2 Session Identifier
SESSION_ID is a unique identifier used by the authorizing entity to
identify the request. It may be used for a number of purposes,
including replay detection, or to correlate this request to a policy
decision entry made by the authorizing entity. For example, the
SESSION_ID can be based on simple sequence numbers or on a standard
NTP timestamp.
+-------+-------+-------+-------+
| Length |X-Type |SubType|
+-------+-------+-------+-------+
| OctetString ...
+-------+-------+-------+-------+
Length
Length of the attribute, which MUST be > 4.
X-Type
SESSION_ID
SubType
No subtypes for SESSION_ID are currently defined; this field MUST
be set to zero. The authorizing entity is the only network entity
that needs to interpret the contents of the SESSION_ID therefore
the contents and format are implementation dependent.
OctetString
Contains the session identifier.
3.3.3 Source Address
SOURCE_ADDR is used to identify the source address specification of
the authorized session. This X-Type may be useful in some scenarios
to make sure the resource request has been authorized for that
particular source address and/or port.
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RFC 3520 Session Authorization Policy Element April 2003
+-------+-------+-------+-------+
| Length |X-Type |SubType|
+-------+-------+-------+-------+
| OctetString ...
+-------+-------+-------+-------+
Length
Length of the attribute, which MUST be > 4.
X-Type
SOURCE_ADDR
SubType
The following sub types for SOURCE_ADDR are defined. IANA acts as
a registry for SOURCE_ADDR sub-types as described in section 7,
IANA Considerations. Initially, the registry contains the
following sub types for SOURCE_ADDR:
1 IPV4_ADDRESS IPv4 address represented in 32 bits
2 IPV6_ADDRESS IPv6 address represented in 128 bits
3 UDP_PORT_LIST list of UDP port specifications,
represented as 16 bits per list entry.
4 TCP_PORT_LIST list of TCP port specifications,
represented as 16 bits per list entry.
OctetString
The OctetString contains the source address information.
In scenarios where a source address is required (see Section 5), at
least one of the subtypes 1 through 2 (inclusive) MUST be included in
every Session Authorization Data Policy Element. Multiple
SOURCE_ADDR attributes MAY be included if multiple addresses have
been authorized. The source address field of the resource
reservation datagram (e.g., RSVP PATH) MUST match one of the
SOURCE_ADDR attributes contained in this Session Authorization Data
Policy Element.
At most, one instance of subtype 3 MAY be included in every Session
Authorization Data Policy Element. At most, one instance of subtype
4 MAY be included in every Session Authorization Data Policy Element.
Inclusion of a subtype 3 attribute does not prevent inclusion of a
subtype 4 attribute (i.e., both UDP and TCP ports may be authorized).
If no PORT attributes are specified, then all ports are considered
valid; otherwise, only the specified ports are authorized for use.
Hamer, et al. Standards Track [Page 8]

RFC 3520 Session Authorization Policy Element April 2003
Every source address and port list must be included in a separate
SOURCE_ADDR attribute.
3.3.4 Destination Address
DEST_ADDR is used to identify the destination address of the
authorized session. This X-Type may be useful in some scenarios to
make sure the resource request has been authorized for that
particular destination address and/or port.
+-------+-------+-------+-------+
| Length |X-Type |SubType|
+-------+-------+-------+-------+
| OctetString ...
+-------+-------+-------+-------+
Length
Length of the attribute, which MUST be > 4.
X-Type
DEST_ADDR
SubType
The following sub types for DEST_ADDR are defined. IANA acts as a
registry for DEST_ADDR sub-types as described in section 7, IANA
Considerations. Initially, the registry contains the following
sub types for DEST_ADDR:
1 IPV4_ADDRESS IPv4 address represented in 32 bits
2 IPV6_ADDRESS IPv6 address represented in 128 bits
3 UDP_PORT_LIST list of UDP port specifications,
represented as 16 bits per list entry.
4 TCP_PORT_LIST list of TCP port specifications,
represented as 16 bits per list entry.
OctetString
The OctetString contains the destination address specification.
In scenarios where a destination address is required (see Section 5),
at least one of the subtypes 1 through 2 (inclusive) MUST be included
in every Session Authorization Data Policy Element. Multiple
DEST_ADDR attributes MAY be included if multiple addresses have been
authorized. The destination address field of the resource
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RFC 3520 Session Authorization Policy Element April 2003
reservation datagram (e.g., RSVP PATH) MUST match one of the
DEST_ADDR attributes contained in this Session Authorization Data
Policy Element.
At most, one instance of subtype 3 MAY be included in every Session
Authorization Data Policy Element. At most, one instance of subtype
4 MAY be included in every Session Authorization Data Policy Element.
Inclusion of a subtype 3 attribute does not prevent inclusion of a
subtype 4 attribute (i.e., both UDP and TCP ports may be authorized).
If no PORT attributes are specified, then all ports are considered
valid; otherwise, only the specified ports are authorized for use.
Every destination address and port list must be included in a
separate DEST_ADDR attribute.
3.3.5 Start time
START_TIME is used to identify the start time of the authorized
session and can be used to prevent replay attacks. If the
AUTH_SESSION policy element is presented in a resource request, the
network SHOULD reject the request if it is not received within a few
seconds of the start time specified.
+-------+-------+-------+-------+
| Length |X-Type |SubType|
+-------+-------+-------+-------+
| OctetString ...
+-------+-------+-------+-------+
Length
Length of the attribute, which MUST be > 4.
X-Type
START_TIME
SubType
The following sub types for START_TIME are defined. IANA acts as
a registry for START_TIME sub-types as described in section 7,
IANA Considerations. Initially, the registry contains the
following sub types for START_TIME:
1 NTP_TIMESTAMP NTP Timestamp Format as defined in
RFC 1305.
OctetString
The OctetString contains the start time.
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RFC 3520 Session Authorization Policy Element April 2003
3.3.6 End time
END_TIME is used to identify the end time of the authorized session
and can be used to limit the amount of time that resources are
authorized for use (e.g., in prepaid session scenarios).
+-------+-------+-------+-------+
| Length |X-Type |SubType|
+-------+-------+-------+-------+
| OctetString ...
+-------+-------+-------+-------+
Length
Length of the attribute, which MUST be > 4.
X-Type
END_TIME
SubType
The following sub types for END_TIME are defined. IANA acts as a
registry for END_TIME sub-types as described in section 7, IANA
Considerations. Initially, the registry contains the following
sub types for END_TIME:
1 NTP_TIMESTAMP NTP Timestamp Format as defined in
RFC 1305.
OctetString
The OctetString contains the end time.
3.3.7 Resources Authorized
RESOURCES is used to define the characteristics of the authorized
session. This X-Type may be useful in some scenarios to specify the
specific resources authorized to ensure the request fits the
authorized specifications.
+-------+-------+-------+-------+
| Length |X-Type |SubType|
+-------+-------+-------+-------+
| OctetString ...
+-------+-------+-------+-------+
Length
Length of the attribute, which MUST be > 4.
X-Type
RESOURCES
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RFC 3520 Session Authorization Policy Element April 2003
SubType
The following sub-types for RESOURCES are defined. IANA acts as a
registry for RESOURCES sub-types as described in section 7, IANA
Considerations. Initially, the registry contains the following
sub types for RESOURCES:
1 BANDWIDTH Maximum bandwidth (kbps) authorized.
2 FLOW_SPEC Flow spec specification as defined in RFC 2205.
3 SDP SDP Media Descriptor as defined in RFC 2327.
4 DSCP Differentiated services codepoint as defined in
RFC 2474.
OctetString
The OctetString contains the resources specification.
In scenarios where a resource specification is required (see Section
5), at least one of the subtypes 1 through 4 (inclusive) MUST be
included in every Session Authorization Data Policy Element.
Multiple RESOURCE attributes MAY be included if multiple types of
resources have been authorized (e.g., DSCP and BANDWIDTH).
3.3.8 Authentication data
The AUTHENTICATION_DATA attribute contains the authentication data of
the AUTH_SESSION policy element and signs all the data in the policy
element up to the AUTHENTICATION_DATA. If the AUTHENTICATION_DATA
attribute has been included in the AUTH_SESSION policy element, it
MUST be the last attribute in the list. The algorithm used to
compute the authentication data depends on the AUTH_ENT_ID SubType
field. See Section 4 entitled Integrity of the AUTH_SESSION policy
element.
A summary of AUTHENTICATION_DATA attribute format is described below.
+-------+-------+-------+-------+
| Length |X-Type |SubType|
+-------+-------+-------+-------+
| OctetString ...
+-------+-------+-------+-------+
Length
Length of the attribute, which MUST be > 4.
X-Type
AUTHENTICATION_DATA
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RFC 3520 Session Authorization Policy Element April 2003
SubType
No sub types for AUTHENTICATION_DATA are currently defined. This
field MUST be set to 0.
OctetString
The OctetString contains the authentication data of the
AUTH_SESSION.
4. Integrity of the AUTH_SESSION policy element
This section describes how to ensure the integrity of the policy
element is preserved.
4.1 Shared symmetric keys
In shared symmetric key environments, the AUTH_ENT_ID MUST be of
subtypes: IPV4_ADDRESS, IPV6_ADDRESS, FQDN, ASCII_DN, UNICODE_DN or
URI. An example AUTH_SESSION policy element is shown below.
+--------------+--------------+--------------+--------------+
| Length | P-type = AUTH_SESSION |
+--------------+--------------+--------------+--------------+
| Length |SESSION_ID | zero |
+--------------+--------------+--------------+--------------+
| OctetString (The session identifier) ...
+--------------+--------------+--------------+--------------+
| Length | AUTH_ENT_ID | IPV4_ADDRESS |
+--------------+--------------+--------------+--------------+
| OctetString (The authorizing entity's Identifier) ...
+--------------+--------------+--------------+--------------+
| Length |AUTH DATA. | zero |
+--------------+--------------+--------------+--------------+
| KEY_ID |
+--------------+--------------+--------------+--------------+
| OctetString (Authentication data) ...
+--------------+--------------+--------------+--------------+
4.1.1 Operational Setting using shared symmetric keys
This assumes both the Authorizing Entity and the Network router/PDP
are provisioned with shared symmetric keys and with policies
detailing which algorithm to be used for computing the authentication
data along with the expected length of the authentication data for
that particular algorithm.
Key maintenance is outside the scope of this document, but
AUTH_SESSION implementations MUST at least provide the ability to
manually configure keys and their parameters locally. The key used
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RFC 3520 Session Authorization Policy Element April 2003
to produce the authentication data is identified by the AUTH_ENT_ID
field. Since multiple keys may be configured for a particular
AUTH_ENT_ID value, the first 32 bits of the AUTH_DATA field MUST be a
key ID to be used to identify the appropriate key. Each key must
also be configured with lifetime parameters for the time period
within which it is valid as well as an associated cryptographic
algorithm parameter specifying the algorithm to be used with the key.
At a minimum, all AUTH_SESSION implementations MUST support the
HMAC-MD5-128 [RFC-2104], [RFC-1321] cryptographic algorithm for
computing the authentication data. New algorithms may be added by
the IETF standards process.
It is good practice to regularly change keys. Keys MUST be
configurable such that their lifetimes overlap allowing smooth
transitions between keys. At the midpoint of the lifetime overlap
between two keys, senders should transition from using the current
key to the next/longer-lived key. Meanwhile, receivers simply accept
any identified key received within its configured lifetime and reject
those that are not.
4.2 Kerberos
In a Kerberos environment, the AUTH_ENT_ID MUST be of the subtype
KRB_PRINCIPAL. The KRB_PRINCIPAL field is defined as the Fully
Qualified Kerberos Principal name of the authorizing entity.
Kerberos [RFC-1510] authentication uses a trusted third party (the
Kerberos Distribution Center - KDC) to provide for authentication of
the AUTH_SESSION to a network server. It is assumed that a KDC is
present and both host and verifier of authentication information
(authorizing entity and router/PDP) implement Kerberos
authentication.
An example of the Kerberos AUTH_DATA policy element is shown below.
+--------------+--------------+--------------+--------------+
| Length | P-type = AUTH_SESSION |
+--------------+--------------+--------------+--------------+
| Length |SESSION_ID | zero |
+--------------+--------------+--------------+--------------+
| OctetString (The session identifier) ...
+--------------+--------------+--------------+--------------+
| Length | AUTH_ENT_ID | KERB_P. |
+--------------+--------------+--------------+--------------+
| OctetString (The principal@realm name) ...
+--------------+--------------+--------------+--------------+
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RFC 3520 Session Authorization Policy Element April 2003
4.2.1. Operational Setting using Kerberos
An authorizing entity is configured to construct the AUTH_SESSION
policy element that designates use of the Kerberos authentication
method (KRB_PRINCIPAL) as defined in RFC 1510. Upon reception of the
resource reservation request, the router/PDP contacts the local KDC,
with a KRB_AS_REQ message, to request credentials for the authorizing
entity (principal@realm). In this request, the client (router/PDP)
sends (in cleartext) its own identity and the identity of the server
(the authorizing entity taken from the AUTH_ENT_ID field) for which
it is requesting credentials. The local KDC responds with these
credentials in a KRB_AS_REP message, encrypted in the client's key.
The credentials consist of 1) a "ticket" for the server and 2) a
temporary encryption key (often called a "session key"). The
router/PDP uses the ticket to access the authorizing entity with a
KRB_AP_REQ message. The session key (now shared by the router/PDP
and the authorizing entity) is used to authenticate the router/PDP,
and is used to authenticate the authorizing entity. The session key
is an encryption key and is also used to encrypt further
communication between the two parties. The authorizing entity
responds by sending a concatenated message of a KRB_AP_REP and a
KRB_SAFE. The KRB_AP_REP is used to authenticate the authorizing
entity. The KRB_SAFE message contains the authentication data in the
safe-body field. The authentication data must be either a 16 byte
MD5 hash or 20 byte SHA-1 hash of all data in the AUTH_SESSION policy
element up to the AUTHENTICATION_DATA (note that when using Kerberos
the AUTH_SESSION PE should not include AUTHENTICATION_DATA as this is
sent in the KRB_SAFE message). The router/PDP independently computes
the hash, and compares it with the received hash in the user-data
field of the KRB-SAFE-BODY [RFC-1510].
At a minimum, all AUTH_SESSION implementations using Kerberos MUST
support the Kerberos des-cbc-md5 encryption type [RFC-1510] (for
encrypted data in tickets and Kerberos messages) and the Kerberos
rsa-md5-des checksum type [RFC-1510] (for the KRB_SAFE checksum)
checksum. New algorithms may be added by the IETF standards process.
Triple-DES encryption is supported in many Kerberos implementations
(although not specified in [RFC-1510]), and SHOULD be used over
single DES.
For cases where the authorizing entity is in a different realm (i.e.,
administrative domain, organizational boundary), the router/PDP needs
to fetch a cross-realm Ticket Granting Ticket (TGT) from its local
KDC. This TGT can be used to fetch authorizing entity tickets from
the KDC in the remote realm. Note that for performance
considerations, tickets are typically cached for extended periods.
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RFC 3520 Session Authorization Policy Element April 2003
4.3.1.1 X.509 V3 digital certificates
When the AUTH_ENT_ID is of type X509_V3_CERT, AUTHENTICATION_DATA
MUST be generated following these steps:
- A Signed-data is constructed as defined in section 5 of CMS
[RFC-3369]. A digest is computed on the content (as specified in
section 6.1) with a signer-specific message-digest algorithm. The
certificates field contains the chain of authorizing entity's
X.509 V3 digital certificates. The certificate revocation list is
defined in the crls field. The digest output is digitally signed
following section 8 of RFC 3447, using the signer's private key.
When the AUTH_ENT_ID is of type X509_V3_CERT, verification MUST be
done following these steps:
- Parse the X.509 V3 certificate to extract the distinguished name
of the issuer of the certificate.
- Certification Path Validation is performed as defined in section 6
of RFC 3280.
- Parse through the Certificate Revocation list to verify that the
received certificate is not listed.
- Once the X.509 V3 certificate is validated, the public key of the
authorizing entity can be extracted from the certificate.
- Extract the digest algorithm and the length of the digested data
by parsing the CMS signed-data.
- The recipient independently computes the message digest. This
message digest and the signer's public key are used to verify the
signature value.
This verification ensures integrity, non-repudiation and data origin.
4.3.1.2 PGP digital certificates
When the AUTH_ENT_ID is of type PGP_CERT, AUTHENTICATION_DATA MUST be
generated following these steps:
- AUTHENTICATION_DATA contains a Signature Packet as defined in
section 5.2.3 of RFC 2440. In summary:
- Compute the hash of all data in the AUTH_SESSION policy element
up to the AUTHENTICATION_DATA.
- The hash output is digitally signed following section 8 of
RFC 3447, using the signer's private key.
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RFC 3520 Session Authorization Policy Element April 2003
When the AUTH_ENT_ID is of type PGP_CERT, verification MUST be done
following these steps:
- Validate the certificate.
- Once the PGP certificate is validated, the public key of the
authorizing entity can be extracted from the certificate.
- Extract the hash algorithm and the length of the hashed data by
parsing the PGP signature packet.
- The recipient independently computes the message digest. This
message digest and the signer's public key are used to verify the
signature value.
This verification ensures integrity, non-repudiation and data origin.
5. Framework
[RFC-3521] describes a framework in which the AUTH_SESSION policy
element may be utilized to transport information required for
authorizing resource reservation for media flows. [RFC-3521]
introduces 4 different models:
1- the coupled model
2- the associated model with one policy server
3- the associated model with two policy servers
4- the non-associated model.
The fields that are required in an AUTH SESSION policy element
dependent on which of the models is used.
5.1 The coupled model
In the Coupled Model, the only information that MUST be included in
the policy element is the SESSION_ID; it is used by the Authorizing
Entity to correlate the resource reservation request with the media
authorized during session set up. Since the End Host is assumed to
be untrusted, the Policy Server SHOULD take measures to ensure that
the integrity of the SESSION_ID is preserved in transit; the exact
mechanisms to be used and the format of the SESSION_ID are
implementation dependent.
5.2 The associated model with one policy server
In this model, the contents of the AUTH_SESSION policy element MUST
include:
- A session identifier - SESSION_ID. This is information that the
authorizing entity can use to correlate the resource reservation
request with the media authorized during session set up.
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RFC 3520 Session Authorization Policy Element April 2003
- The identity of the authorizing entity - AUTH_ENT_ID. This
information is used by the Edge Router to determine which
authorizing entity (Policy Server) should be used to solicit
resource policy decisions.
In some environments, an Edge Router may have no means for
determining if the identity refers to a legitimate Policy Server
within its domain. In order to protect against redirection of
authorization requests to a bogus authorizing entity, the
AUTH_SESSION MUST also include:
- AUTHENTICATION_DATA. This authentication data is calculated over
all other fields of the AUTH_SESSION policy element.
5.3 The associated model with two policy servers
The content of the AUTH_SESSION Policy Element is identical to the
associated model with one policy server.
5.4 The non-associated model
In this model, the AUTH_SESSION MUST contain sufficient information
to allow the Policy Server to make resource policy decisions
autonomously from the authorizing entity. The policy element is
created using information about the session by the authorizing
entity. The information in the AUTH_SESSION policy element MUST
include:
- Calling party IP address or Identity (e.g., FQDN) - SOURCE_ADDR
X-TYPE
- Called party IP address or Identity (e.g., FQDN) - DEST_ADDR
X-TYPE
- The characteristics of (each of) the media stream(s) authorized
for this session - RESOURCES X-TYPE
- The authorization lifetime - START_TIME X-TYPE
- The identity of the authorizing entity to allow for validation of
the token in shared symmetric key and Kerberos schemes -
AUTH_ENT_ID X-TYPE
- The credentials of the authorizing entity in a public-key
scheme - AUTH_ENT_ID X-TYPE
- Authentication data used to prevent tampering with the
AUTH_SESSION policy element - AUTHENTICATION_DATA
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RFC 3520 Session Authorization Policy Element April 2003
Furthermore, the AUTH_SESSION policy element MAY contain:
- The lifetime of (each of) the media stream(s) - END_TIME X-TYPE
- Calling party port number - SOURCE_ADDR X-TYPE
- Called party port number - DEST_ADDR X-TYPE
All AUTH_SESSION fields MUST match with the resource request. If a
field does not match, the request SHOULD be denied.
6. Message Processing Rules
6.1 Generation of the AUTH_SESSION by the authorizing entity
1. Generate the AUTH_SESSION policy element with the appropriate
contents as specified in section 5.
2. If authentication is needed, the entire AUTH_SESSION policy
element is constructed, excluding the length, type and subtype
fields of the AUTH_SESSION field. Note that the message MUST
include either a START_TIME or a SESSION_ID (See Section 9), to
prevent replay attacks. The output of the authentication
algorithm, plus appropriate header information, is appended to the
AUTH_SESSION policy element.
6.2 Message Generation (RSVP Host)
An RSVP message is created as specified in [RFC-2205] with the
following modifications.
1. RSVP message MUST contain at most one AUTH_SESSION policy element.
2. The AUTH SESSION policy element received from the authorizing
entity (Section 3.2) MUST be copied without modification into the
POLICY DATA object.
3. POLICY_DATA object (containing the AUTH_SESSION policy element) is
inserted in the RSVP message in the appropriate place.
6.3 Message Reception (RSVP-aware Router)
RSVP message is processed as specified in [RFC-2205] with following
modifications.
1. If router is policy aware then it SHOULD send the RSVP message to
the PDP and wait for response. If the router is policy unaware
then it ignores the policy data objects and continues processing
the RSVP message.
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RFC 3520 Session Authorization Policy Element April 2003
2. Reject the message if the response from the PDP is negative.
3. Continue processing the RSVP message.
6.4 Authorization (Router/PDP)
1. Retrieve the AUTH_SESSION policy element. Check the PE type field
and return an error if the identity type is not supported.
2. Verify the message integrity.
- Shared symmetric key authentication: The Network router/PDP
uses the AUTH_ENT_ID field to consult a table keyed by that
field. The table should identify the cryptographic
authentication algorithm to be used along with the expected
length of the authentication data and the shared symmetric key
for the authorizing entity. Verify that the indicated length
of the authentication data is consistent with the configured
table entry and validate the authentication data.
- Public Key: Validate the certificate chain against the trusted
Certificate Authority (CA) and validate the message signature
using the public key.
- Kerberos Ticket: If the AUTH_ENT_ID is of subtype
KRB_PRINCIPAL, Request a ticket for the authorizing entity
(principal@realm) from the local KDC. Use the ticket to access
the authorizing entity and obtain authentication data for the
message.
3. Once the identity of the authorizing entity and the validity of
the service request has been established, the authorizing
router/PDP MUST then consult its local policy tables (the contents
of which are a local matter) in order to determine whether or not
the specific request is authorized. To the extent to which these
access control decisions require supplementary information,
routers/PDPs MUST ensure that supplementary information is
obtained securely. An example of insecure access control
decisions would be if the authorizing party relies upon an
insecure database (such as DNS or a public LDAP directory) and
authorizes with a certificate or an FQDN.
4. Verify the requested resources do not exceed the authorized QoS.
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RFC 3520 Session Authorization Policy Element April 2003
7. Error Signaling
If a PDP fails to verify the AUTH_SESSION policy element then it MUST
return a policy control failure (Error Code = 02) to the PEP. The
error values are described in [RFC-2205] and [RFC-2750]. Also the
PDP SHOULD supply a policy data object containing an AUTH_DATA Policy
Element with A-Type=POLICY_ERROR_CODE containing more details on the
Policy Control failure [RFC-3182]. If RSVP is being used, the PEP
MUST include this Policy Data object in the outgoing RSVP Error
message.
8. IANA Considerations
Following the policies outlined in [IANA-CONSIDERATIONS], Standard
RSVP Policy Elements (P-type values) are assigned by IETF Consensus
action as described in [RFC-2750].
P-Type AUTH_SESSION is assigned the value 0x04.
Following the policies outlined in [IANA-CONSIDERATIONS], session
authorization attribute types (X-Type)in the range 0-127 are
allocated through an IETF Consensus action; X-Type values between
128-255 are reserved for Private Use and are not assigned by IANA.
X-Type AUTH_ENT_ID is assigned the value 1.
X-Type SESSION_ID is assigned the value 2.
X-Type SOURCE_ADDR is assigned the value 3.
X-Type DEST_ADDR is assigned the value 4.
X-Type START_TIME is assigned the value 5.
X-Type END_TIME is assigned the value 6.
X-Type RESOURCES is assigned the value 7.
X-Type AUTHENTICATION_DATA is assigned the value 8.
Following the policies outlined in [IANA-CONSIDERATIONS],
AUTH_ENT_ID SubType values in the range 0-127 are allocated through
an IETF Consensus action; SubType values between 128-255 are
reserved for Private Use and are not assigned by IANA.
AUTH_ENT_ID SubType IPV4_ADDRESS is assigned the value 1.
SubType IPV6_ADDRESS is assigned the value 2.
SubType FQDN is assigned the value 3.
SubType ASCII_DN is assigned the value 4.
SubType UNICODE_DN is assigned the value 5.
SubType URI is assigned the value 6.
SubType KRB_PRINCIPAL is assigned the value 7.
SubType X509_V3_CERT is assigned the value 8.
SubType PGP_CERT is assigned the value 9.
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RFC 3520 Session Authorization Policy Element April 2003
Following the policies outlined in [IANA-CONSIDERATIONS],
SOURCE_ADDR SubType values in the range 0-127 are allocated through
an IETF Consensus action; SubType values between 128-255 are
reserved for Private Use and are not assigned by IANA.
SOURCE_ADDR SubType IPV4_ADDRESS is assigned the value 1.
SubType IPV6_ADDRESS is assigned the value 2.
SubType UDP_PORT_LIST is assigned the value 3.
SubType TCP_PORT_LIST is assigned the value 4.
Following the policies outlined in [IANA-CONSIDERATIONS],
DEST_ADDR SubType values in the range 0-127 are allocated through an
IETF Consensus action; SubType values between 128-255 are reserved
for Private Use and are not assigned by IANA.
DEST_ADDR SubType IPV4_ADDRESS is assigned the value 1.
SubType IPV6_ADDRESS is assigned the value 2.
SubType UDP_PORT_LIST is assigned the value 3.
SubType TCP_PORT_LIST is assigned the value 4.
Following the policies outlined in [IANA-CONSIDERATIONS],
START_TIME SubType values in the range 0-127 are allocated through an
IETF Consensus action; SubType values between 128-255 are
reserved for Private Use and are not assigned by IANA.
START_TIME SubType NTP_TIMESTAMP is assigned the value 1.
Following the policies outlined in [IANA-CONSIDERATIONS],
END_TIME SubType values in the range 0-127 are allocated through an
IETF Consensus action; SubType values between 128-255 are reserved
for Private Use and are not assigned by IANA.
END_TIME SubType NTP_TIMESTAMP is assigned the value 1.
Following the policies outlined in [IANA-CONSIDERATIONS],
RESOURCES SubType values in the range 0-127 are allocated through an
IETF Consensus action; SubType values between 128-255 are reserved
for Private Use and are not assigned by IANA.
RESOURCES SubType BANDWIDTH is assigned the value 1.
SubType FLOW_SPEC is assigned the value 2.
SubType SDP is assigned the value 3.
SubType DSCP is assigned the value 4.
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RFC 3520 Session Authorization Policy Element April 2003
9. Security Considerations
The purpose of this document is to describe a mechanism for session
authorization to prevent theft of service.
Replay attacks MUST be prevented. In the non-associated model, the
AUTH_SESSION policy element MUST include a START_TIME field and the
Policy Servers MUST support NTP to ensure proper clock
synchronization. Failure to ensure proper clock synchronization will
allow replay attacks since the clocks of the different network
entities may not be in-synch. The start time is used to verify that
the request is not being replayed at a later time. In all other
models, the SESSION_ID is used by the Policy Server to ensure that
the resource request successfully correlates with records of an
authorized session. If a AUTH_SESSION is replayed, it MUST be
detected by the policy server (using internal algorithms) and the
request MUST be rejected.
To ensure that the integrity of the policy element is preserved in
untrusted environments, the AUTHENTICATION_DATA attribute MUST be
included.
In environments where shared symmetric keys are possible, they should
be used in order to keep the AUTH_SESSION policy element size to a
strict minimum. This is especially true in wireless environments
where the AUTH_SESSION policy element is sent
over-the-air. The shared symmetric keys authentication option MUST
be supported by all AUTH_SESSION implementations.
If shared symmetric keys are not a valid option, the Kerberos
authentication mechanism is reasonably well secured and efficient in
terms of AUTH_SESSION size. The AUTH_SESSION only needs to contain
the principal@realm name of the authorizing entity. This is much
more efficient than the PKI authentication option.
PKI authentication option provides a high level of security and good
scalability, however it requires the presence of credentials in the
AUTH_SESSION policy element which impacts its size.
10. Acknowledgments
We would like to thank Francois Audet, Don Wade, Hamid Syed, Kwok Ho
Chan and many others for their valuable comments. Special thanks to
Eric Rescorla who provided numerous comments and suggestions that
improved this document.
In addition, we would like to thank S. Yadav, et al., for their
efforts on RFC 3182, as this document borrows from their work.
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RFC 3520 Session Authorization Policy Element April 2003
12. Informative References
[IANA-CONSIDERATIONS] Alvestrand, H. and T. Narten, "Guidelines for
Writing an IANA Considerations Section in
RFCs", BCP 26, RFC 2434, October 1998.
[RFC-3261] Rosenberg, J., Schulzrinne, H., Camarillo, G.,
Johnston, A., Peterson, J., Sparks, R.,
Handley, M. and E. Schooler, "SIP: Session
Initiation Protocol", RFC 3261, June 2002.
13. Intellectual Property Statement
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pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
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has made any effort to identify any such rights. Information on the
IETF's procedures with respect to rights in standards-track and
standards-related documentation can be found in BCP-11. Copies of
claims of rights made available for publication and any assurances of
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obtain a general license or permission for the use of such
proprietary rights by implementors or users of this specification can
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The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights which may cover technology that may be required to practice
this standard. Please address the information to the IETF Executive
Director.
Hamer, et al. Standards Track [Page 27]

RFC 3520 Session Authorization Policy Element April 2003
16. Full Copyright Statement
Copyright (C) The Internet Society (2003). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
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included on all such copies and derivative works. However, this
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The limited permissions granted above are perpetual and will not be
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
Hamer, et al. Standards Track [Page 30]